The Chelopech region consists of a Precambrian metamorphic basement consisting of gneisses, amphibolites, and metasediments overlain by Upper Cretaceous, volcano-sedimentary sequences which include the Chelopech Formation; the primary host to mineralisation. The Chelopech Formation reaches thicknesses of up to 2,000 m and consists of Lower and Upper units.

Mineralisation is hosted within the Lower Chelopech Formation and is characterised by typical epithermal, high-sulphidation (HS) alteration. Alteration and mineralisation are typically zonal with central, high-grade units associated with well-developed stockworks and massive sulphide mineralisation. These units are surrounded by lower-grade haloes dominated by disseminated sulphides and pervasive silica overprinting. These two zones are respectively referred to as “Stockwork” and “Silica Envelopes” and are used as hard boundaries during the estimation of Mineral Resources.

Mineralisation
Three successive mineralisation stages have been recognised at Chelopech, including an early iron- sulphur stage consisting mainly of disseminated and massive pyrite, a second copper-arsenic-sulphur stage which is the economic copper and gold stage, and a late lead-zinc stage. These display different geometries, including veins, breccias, massive and disseminated sulphides.

The mineralisation occurs in a range of different morphologies, including lens-like, pipe-like and columnar bodies that typically dip steeply towards the south. The mineralised zones vary from 40 m to 200 m in length, are 20–130 m thick, and can extend at least 390 m down plunge. Sub-vertical vein mineralisation is volumetrically the most important mineralisation style at Chelopech (Chambefort, 2005).

Sulphide mineralogy is dominated by pyrite, marcasite, melnikovite, tennantite, enargite-luzonite, and chalcopyrite, together with subordinate famatinite, sphalerite and galena. In gross terms, about 45% of the copper is in the form of arsenides and sulfosalts, 50% as chalcopyrite and 5% as oxides.

Quartz, barite and kaolinite are the dominant gangue minerals with chlorite, ankerite and gypsum subordinate. Quartz barite-sulphides mineralization with high gold grades and low copper is typical for peripheral zone near the covering sediments (Block 700).

Gold occurs in a variety of forms, both as native metal with admixed silver in a stoichiometric form approximating to Au3Ag and in auriferous tellurides. The gold is fine grained (5–300 microns, with 5–20 microns the norm). Metallurgical studies have shown a significant proportion of the gold is refractory, typically:

• 45% intergrown within pyrite, chalcopyrite and sphalerite;
• 25% intergrown with enargite, luzonite, tennantite, tetrahedrite and bornite;
• 20% finely intergrown with chalcedonic silica;
• 10% as free gold.

Silver-bearing rock and native silver are usually spatially associated or finely intergrown with pyrite and galena (62%) with enargite, tennantite and tetrahedrite (15%) and as electrum (23%).

Other major sulphides and arsenides exhibit simple crystalline and intergrown forms with the pyrite and occur in intra-crystal spaces as replacements, as replacements of pyrite, as cross-cutting veinlets and as overgrowths. Intergrowths of the cupriferous minerals are commonplace, both as aggregates and as complex textures with several intergrown minerals.

Underground mining production is performed using sublevel long-hole open stoping methods. The various orebodies are developed at nominal 30 m vertical intervals and accessed by major declines in both the Western and Central areas. Stopes are designed to be 20 m wide between the levels. The length of the stope depends on the geotechnical conditions, but can range between 20 m and 60 m. The most recent trend of stope design is to keep a 20–30 m length and 60 m height, where geological and geotechnical conditions are suitable. This allows for improvement in ore handling and dust suppression during ore mucking because of shorter remote loading. Ore is delivered via ore passes, or via trucks, to the ROM bin above the crusher. The crusher feeds up to 400 tph to a system of eight conveyors, to transport the ore to the surface stockpile.

Once mined via an “end-slice” methodology, stopes are backfilled with “paste-fill” produced from the mill tailings to which cement is added and which is gravity fed underground via a system of borehole and pipes to the stopes being filled.

Multiple horizons are designed in each ore body so that multiple stopes can be in production at any one time. Simulations have shown that at least six stopes shall need to be producing ore to maintain ore production of 2.2 Mtpa, with up to 22 stopes being drilled, “mucked” and filled at any one time.

Mine Ventilation
The ventilation system has been in a stable configuration since the last major upgrade in 2014 which saw installation of four 110 kW fans working in parallel at Zapad shaft on 405 level. The Zapad shaft is 3.5 m diameter, bare concrete-lined shaft, which was stripped after its decommissioning as ore hoisting shaft.

Backfill
A paste backfill plant has been built on surface, commissioned in 2010, to facilitate maximum use of the available tailings for backfill placement underground in the mine. This will meet future backfill requirements and has replaced the existing hydraulic backfill plant. The facility is built adjacent to the existing hydraulic backfill plant and makes use of existing binder silos and backfill reticulation holes.

The paste backfill plant consists of a high-rate thickener, vacuum filter, mixer and binder addition system. A complete underground borehole and piping paste reticulation system has been installed with the plant, having a capacity of producing 230 tph of paste backfill.

Target design strengths for the paste for stope filling range between 260 kPa and 450 kPa after 56 days. The required strength is dependent on the location of the fill in the stope. Cement contents typically range between 3.5% and 5%. A QAQC program for paste-fill strength determination is in place run by the geotechnical team. Optimisation of the process will continue to be an ongoing process.

Dry waste material from waste developments is used to backfill stopes where paste-fill is not required and typically constitutes around 15% of the total stope backfill volume.

Underground Crusher Conveyor System
A materials handling system for the mine was designed by DPMC and constructed to replace the earlier shaft and rail ore handling system in 2012 . The ore reports via an ore pass system to the 195 level where it is then crushed and transported by a series of six conveyors (3.9 km total length) to be finally discharged onto a 6,000-tonne live capacity reclaim stockpile on surface. The new system has a 3 Mtpa maximum capacity.

A materials handling system for the mine was designed by DPMC and constructed to replace the earlier shaft and rail ore handling system in 2012 (discussed further in Section 18). The ore reports via an ore pass system to the 195 level where it is then crushed and transported by a series of six conveyors (3.9 km total length) to be finally discharged onto a 6,000-tonne live capacity reclaim stockpile on surface. The new system has a 3 Mtpa maximum capacity.

This ore handling system incorporates a primary crusher (a 1,070 mm x 1,500 mm jaw crusher) between the 195 level and the 165 level underground, which discharges into a 400-tonne crushed ore bin. The crusher is fed from a ROM bin sitting under a grizzly with openings of 800 mm x 800 mm.

Ore is fed to the grizzly via three sources:
1. A 4 m diameter x 135 m long ore pass for 151 and 150 block material above the 260 level.
2. A 7 m diameter x 30 m long ore bin for the 144, 145, 147, 149, and 103 blocks, 150 and 151 blocks between the 225 and 260 levels; and the Central area 16, 18 and 19 blocks.
3. A truck tip directly on the grizzly for ore in 151 and 150 blocks, on and below the 195 level.

A plate feeder draws material from the 400 t crushed ore bin and loads a picking belt (CV1) for removal of tramp metal using a self-cleaning magnet. Material is then conveyed via six more haulage conveyors (CV2- CV7) to the surface. The surface conveyor (C1105) transfers this material to the surface reclaim stockpile, where it is reclaimed and conveyed to the SAG mill to provide uninterrupted feed to the process plant.

One crusher exists on surface to handle oversize and to supply minimum production in case of emergency.

There is no ore blending ability in the system from the ore passes to the plant delivery. Ore blending is therefore done by controlling the amount of material coming from each producing stope and the planning behind how many and what stopes will be in production.

Crushed product from the primary crushers, which has a typical P80 of 100 mm, is ground using a single-stage closed grinding circuit with cyclone classification. This comprises a single-stage SAG mill, 8.53 m diameter x 4.72 m long, with a rated capacity of 5,800 kW. Cyclone underflow is returned to the SAG mill and the overflow gravitates to the flotation circuit passing via an “in-stream” analysing system, which monitors the density and the assay composition of the stream, and a particle size analyser.

Current ore treatment processes comprise conventional crushing of ROM ore in a primary jaw crushing circuit, grinding in a SAG milling circuit, rougher/scavenger flotation, followed by three-stage cleaning and concentrate dewatering to produce a copper/gold concentrate. Pyrite is recovered from the copper circuit cleaner tails as a by-product with minor gold credits.

The primary saleable product is a gold-copper concentrate, the grade of the concentrate can be altered depending on the client. The concentrate grade that maximises the project value depending on the client and market conditions was determined through DPMC internal studies.

The concentrate is loaded at the mine site through a conveyor system from the stockpile into rail wagons and dispatched to the Port of Burgas for sea transportation to the Company’s smelter in Namibia or other clients in China.

Since 2014, pyrite concentrate, containing gold, has been produced in a section with a capac ........